Empirical and Molecular Formula Notes


The empirical formula of a compound shows the ratio of elements present in a compound
The molecular formula of a compound shows how many atoms of each element are present in a
molecule of a compound.
o Example: the compound butene has a molecular formula of C4H8. The empirical formula
of butene is CH2 because there is a 1:2 ratio of carbon atoms to hydrogen atoms.
Finding the molecular formula when given the empirical formula
 The molecular formula is just a multiple of the empirical formula.
 Take a look at butene:
o CH2 x n = C4H8 so n=4. What is the molar mass of butene?
 If the empirical formula of a compound is CH2, then you can find the molar mass
of the empirical formula: 12.0+ 2(1.0) = 14.0 g/mol
 14.0 g/mol x 4 = 56 g/mol
 What if I have a compound with an empirical formula of CH2 that has a molar mass of 84 g/mol.
What is the molecular formula of this compound?
o 14.0 g/mol x n = 84 g/mol so n = 6
o CH2 x 6 = C6H12
 Try a few:
1. What is the empirical formula of C8H10? C6H12O6?
2. What is the molecular formula of a molecule that has an empirical formula of NO2 and a
molar mass of 92 g/mol?
3. Consider a molecule that has an empirical formula of SO3 and a molecular formula of S3O9.
What is the molar mass of the molecule?
Calculating empirical formula from percent composition
Answer this question:
What is the empirical formula of a compound that contains 25.53% Mg and 74.47% Cl?
Steps:
1. Assume a 100 g sample of the compound
2. Convert all percentages into grams. Example: 50% = 50g/100g, 21% = 21 g
3. Find the atomic mass of each element using the periodic table
4. Calculate the moles of each element present
5. Divide the moles of each element by the smallest number of moles to get a mole ratio
6. If the numbers in this ratio are whole numbers, convert into an empirical formula
7. If the numbers in this ratio are NOT whole numbers, multiply them by a number that will make
them both whole.
Try your own:
A compound with a molecular mass of 34.0 g/mol is known to contain 5.88% H and 94.12% oxygen.
What is the molecular formula for this compound? (Hint: you must find the empirical formula first!!)